The present invention relates to processes for synthesizing borohydride compounds, and more particularly to more efficient processes for synthesizing borohydride compounds that have decreased sodium or alkali metal requirements.
Sodium borohydride is an important reducing agent for many organic chemical functional groups (including aldehydes and ketones) and metal salts with various applications in pharmaceutical and fine chemical manufacturing. It can also be used as a purification agent to remove metal ions from industrial waste streams or carbonyl and peroxide impurities from process chemicals. Aqueous solutions of sodium borohydride are used in the pulp and paper industries to produce sodium hydrosulfite bleach.
In addition, sodium borohydride is being evaluated as a hydrogen source for fuel cells and hydrogen-burning internal combustion engines. Sodium borohydride can be used directly as an anodic fuel in a fuel cell or as a hydrogen storage medium (e.g., hydrogen can be liberated by the reaction of sodium borohydride with water, which produces sodium borate as a byproduct). As with all fuels, acceptance of sodium borohydride in the commercial market is partially dependent on the availability of industrial scale quantities.
Sodium borohydride is commercially prepared from the conversion of boric acid and methanol into trimethyl borate (B(OCH3)3) which is then reduced by sodium hydride to produce sodium borohydride (Equation 1). This process is essentially unchanged from that described in Schlesinger, H. I., Brown, H. C., Abraham, B., Bond, A. C., Davidson, N., Finholt, A. E., Gilbreath, J. R., Hoekstra, H. R., Horvitz, L., Hyde, E. K., Katz, J. J., Knight, J., Lad, R. A., Mayfield, D. L., Rapp, L., Ritter, R. M., Schwartz, A. M., Sheft, I., Tuck, L. D., and Walker, A. O., xe2x80x9cNew Developments in the Chemistry of Diborane and the Borohydrides. I. General Summary,xe2x80x9d Journal of the American Chemical Society, vol. 75 (1953), pp. 187-190 (xe2x80x9cthe Schlesinger processxe2x80x9d).
B(OH)3+CH3OHxe2x86x92B(OCH3)3+4NaHxe2x86x92NaBH4+3NaOCH3xe2x80x83xe2x80x83(1)
The use of large amounts of sodium metal (4 moles needed to produce one mole of sodium borohydride) is a major factor in the manufacturing cost. According to U. S. Geologic Survey reports, the largest single use for metallic sodium in the United States is in sodium borohydride production. Sodium metal is produced by the electrolysis of molten salt mixtures of sodium chloride and calcium chloride in an energy-intensive process. Sodium hydride is prepared on-site by reaction of sodium metal with hydrogen in a mineral oil slurry.
The process described in Equation 1 provides poor molar economy by requiring 4 moles of sodium (as sodium hydride) to produce 1 mole of sodium borohydride. Based on the above stoichiometry, 75% of the sodium required is converted to a by-product, sodium methoxide. This inefficiency limits the scalability of this process. A process that utilizes sodium atoms more efficiently (i.e., more sodium atoms incorporated into desired product) would therefore be desirable. Additionally, both sodium metal and sodium hydride will react violently with water to generate hydrogen gas, and must be protected from all sources of water, usually under an inert gas atmosphere. Special engineering and safety considerations must be made to prevent possible explosive reactions between sodium and water.
The product of Equation 1 is a mineral oil dispersion of sodium borohydride and sodium methoxide. This mixture is hydrolyzed to produce a two-phase aqueous sodium hydroxide-sodium borohydride-methanol mixture; methanol is then removed from this mixture by distillation. The aqueous solution is a major commercial product; however, powder sodium borohydride is the desired reagent for use in pharmaceutical synthesis and for hydrogen generation applications. Additional extraction, evaporation, crystallization, and drying steps are necessary to obtain solid sodium borohydride.
Alternative routes to produce sodium borohydride have been evaluated in an effort to reduce manufacturing costs, but have not replaced the route shown in Equation 1, as discussed in xe2x80x9cNa Borohydride: can cost be lowered?xe2x80x9d, Canadian Chemical Processing, (1963) pp. 57-62. For example. Bayer AG evaluated a solid-state reaction of borax, quartz, and sodium metal (Equation 2) under a hydrogen atmosphere; however, this approach still requires the unfavorable sodium metal stoichiometry of 4 moles per mole of sodium borohydride produced.
Na2B4O7+16Na+8H2+7SiO2xe2x86x924NaBH4+7Na2SiO3xe2x80x83xe2x80x83(2)
Furthermore, in view of the large quantities of sodium borohydride needed for use as a hydrogen carrier, e.g., as an alternative fuel in the transportation industry, these processes would produce large quantities of waste products such as sodium methoxide or sodium silicate. Further energy and expense is required to separate these byproducts.
Most improvements found in the prior art are simple modifications of the Schlesinger and Bayer processes represented by Equations 1 and 2. Accordingly, such improvements also suffer from the disadvantages stated above and do not provide any improved efficiency. The idea process would be one in which the majority of the sodium atoms are converted to sodium borohydride product.
An alternative process which conceptually could be used to prepare sodium borohydride compounds on a large scale is the disproportionation of diborane with small, hard Lewis bases. The use of methoxide and hydroxide bases is discussed, for example, in U.S. Pat. No. 2,461,662, in Schlesinger, H. I., Brown, H. C., Hoekstra, H. R., and Rapp, L. R., xe2x80x9cReactions of Diborane with Alkali Metal Hydrides and Their Addition Compounds. New Syntheses of Borohydridesxe2x80x9d, Journal of the American Chemical Society, vol. 75 (1953), pp. 199-204; and in Davis, R. E., and Gottbrath, J. A., xe2x80x9cOn the Nature of Stock""s Hypoborate,xe2x80x9d Chemistry and Industry, (1961) pp. 1961-1962. Not only does this result in a favorable utilization of sodium atoms as compared to the processes shown in Equations 1 and 2, it would also allow the use of air and moisture stable sodium salts (as compared to sodium hydride), which simplify handling during production.
However, this approach has not been seriously considered for commercial uses. The reaction of diborane with aqueous sodium hydroxide described in the Davis and Gottbrath article referenced above generates a mixture of products, including sodium borohydride. The methodology described in U.S. Pat. No. 2,461,662 and in the Schlesinger, Brown, Hoekstra, and Rapp article referenced above requires that gaseous diborane be condensed onto solid metal alkoxides at low temperature (ca. xe2x88x92100xc2x0 C.).
In view of the above, there is a need for improved and energy efficient industrial scale manufacturing processes for producing borohydride compounds that eliminate the need for excess sodium or alkali metals. In addition, there is a need for industrial scale processes that reduce or avoid the production of large quantities of waste products.
The present invention provides processes for producing large quantities of borohydride compounds, which overcome the above-described deficiencies. In addition, the efficiency of the processes of the present invention can be greatly enhanced over the typical processes for producing borohydride compounds. Further, sodium carbonate (soda ash) is a readily available mined chemical mineral and requires no special handling as compared to metallic sodium or sodium hydride.
In one embodiment of the present invention, a process is provided for producing borohydride compounds which includes the reaction of a carbonate of the formula Y2CO3 in aqueous solution at a temperature of about xe2x88x925 to about 20xc2x0 C. with diborane to produce the borohydride YBH4, where Y is a monovalent cationic moiety.
In another embodiment of the present invention, a process is provided for producing borohydride compounds which includes the reaction of a base with a borane complex, the borane complex in a solution comprising a non-aqueous aprotic solvent or a non-aqueous polar solvent, to produce the borohydride YBH4, where Y is a monovalent cationic moiety and the base is selected from the group consisting of a hydroxide of the formula YOH and a carbonate of the formula Y2CO3.
In another embodiment of the present invention, a process is provided for producing borohydride compounds which includes the reaction of a base in the solid phase with gaseous diborane to produce the borohydride YBH4, where Y is a monovalent cationic moiety and the base is selected from the group consisting of a hydroxide of the formula YOH and a carbonate of the formula Y2CO3. No solvent is used in this embodiment of the invention.
In another embodiment of the present invention, a process is provided for producing borohydride compounds which includes the reaction of a base suspended in a non-aqueous aprotic solvent or a non-aqueous polar solvent with gaseous diborane to produce the borohydride YBH4, where Y is a monovalent cationic moiety and the base is selected from the group consisting of a hydroxide, YOH and a carbonate of the formula Y2CO3.
The stoichiometry of the above reactions can be summarized by the following equations:
Additionally, the processes of this invention may also be integrated into an overall regeneration scheme of large volume sodium borohydride production from boron-containing ores and natural gas, such as the schemes described in U.S. patent application Ser. No. 09/710,041 and U.S. patent application Ser. No. 09/833,904 (herein incorporated by reference).
The present invention includes processes for producing borohydride compounds from diborane complexes. In accordance with the present invention, these processes can be conducted in a batchwise or continuous manner, as is well-known to the skilled artisan.
The processes of the invention substantially reduce the requirement for excess quantities of sodium metal that exists in current industrial processes, thereby decreasing the energy cost commonly associated with borohydride production. About 75% of sodium from the sodium-containing starting material (sodium carbonate or sodium hydroxide) is converted to sodium borohydride in the current invention. In contrast, only about 25% of sodium from the starting materials (sodium hydride, or sodium metal, respectively) in the Schlesinger and Bayer processes is converted into sodium borohydride.
The embodiment of the present invention which utilizes a carbonate base in aqueous solution provides a novel and mild procedure for obtaining sodium borohydride. The other embodiments of the present invention describe novel reactions in non-aqueous solvents or heterogeneous reactions between a gas and a solid. The preferred non-aqueous embodiments of the invention result in high yields by avoiding an alternative reaction pathway available to diborane in the presence of water as well as the potential decomposition of the desired sodium borohydride product by water.
In one exemplary embodiment of the invention, the reaction of a carbonate in aqueous solution with diborane is performed using standard Schlenk glassware and techniques for manipulating air-sensitive materials, incorporating a dispersion tube submerged in an aqueous solution kept at about xe2x88x925 to about 20xc2x0 C., preferably about 0 to about 5xc2x0 C., and most preferably at about 0xc2x0 C. A gas inlet tube submerged in the aqueous solution may be used instead of the dispersion tube.
In another exemplary embodiment, between about one and about five molar equivalents of the carbonate may be provided in the reaction medium for every molar equivalent of diborane.
In another exemplary embodiment of the invention, the reaction of the carbonate in aqueous solution with diborane is performed in multiple reaction flasks containing sodium carbonate solutions connected in sequence, and the diborane gas stream bubbled though each flask in turn. The reaction may be performed in batch mode or in continuous mode. Preferably, the continuous mode reaction employs a gas recycle pump which can be incorporated into the reaction apparatus to recirculate the diborane through the reaction flasks.
In another exemplary embodiment of the invention, the reaction of a hydroxide or of a carbonate base in diethylene glycol dimethyl ether (diglyme) with a borane complex is performed in standard chemical glassware with stirring by a magnetic stirrer.
In one preferred embodiment, the borane complex and the base are reacted at a temperature between about xe2x88x925 and about 30xc2x0 C., most preferably between about 20 and about 25xc2x0 C. In another preferred embodiment, the reaction time is preferably between 2 and 70 hours, most preferably between 24 and 65 hours.
In other exemplary embodiments of the invention, the reactions where one of the reactants is gaseous diborane and the other reactant is a hydroxide or a carbonate, and where the other reactant is either in the absence of solvent or suspended in diglyme, are carried out in a ballmill vessel. The vessel is rotated by a mill while milling media placed in the interior of the vessel is used to agitate the contents of the vessel. The milling media may be of any suitable nonreactive substance including, but not limited to, steel, copper, aluminum, tungsten carbide, brass thermoplastic or ceramic balls.
In one preferred embodiment, diborane and the base are reacted at a temperature between about 0 and about 70xc2x0 C., most preferably between about 20 and about 25xc2x0 C. In another preferred embodiment, diborane and base are reacted at pressures between about 14 psi and 200 psi, preferably between 14 and 100 psi. In another preferred embodiment, the reaction time is preferably between 2 and 100 hours, most preferably between 40 and 65 hours. In still another preferred embodiment, for every molar equivalent of diborane, between about one and about ten molar equivalents of the base may be provided in the reaction medium, preferably between about five and about ten molar equivalents of the base, most preferably between about ten molar equivalents of the base. Where solid sodium carbonate is used as the basic reactant, following isolation of sodium borohydride at the completion of the reaction, the remaining solid byproducts can be extracted by adding alcohol, preferably methanol, to remove sodium borate, which is soluble in the alcohol, from sodium carbonate, which is insoluble in the alcohol. The recovered solid sodium carbonate can be recycled for use in future disproportionation reactions.
The reactants Y2CO3 and YOH are readily available commercially from Aldrich Chemical Co., Inc. Preferably, Y is an alkali metal, a pseudo-alkali metal such as Tl+, an ammonium ion, a quaternary ammonium ion of formula NR4+, wherein R is hydrogen, a straight chain C1 to C4 alkyl group, a branched chain C1 to C4 alkyl group, or a mixture thereof. More preferably, Y is Li+, Na+, K+, Rb+, Cs+, Fr+, NH4+, Tl+, or a quaternary ammonium ion of formula NR4+, wherein R is hydrogen, a straight chain C1 to C4 alkyl group, or a branched chain C1 to C4 alkyl group. Most preferably, Y is Na+, Li+, or K+.
The reactants diborane and borane complexes are also readily available commercially from Aldrich Chemical Co., Inc. Preferably, borane complexes are borane complexes of ethers, borane-morpholine, borane-ammonia, borane-triethylamine, borane-trimethylamine, borane-N,N-diethylaniline, borane-dimethylamine, borane-dimethylsulfide, borane-inert-butylamine, borane-diethylamine, borane-iso-propylamine and borane-pyridine. Most preferably, the borane complexes are borane-tetrahydrofuran and borane-triethylamine.
Without wishing to be bound by any theory, it is believed that small hard bases promote the asymmetric dissociation of diborane, as described, for example, in Huheey, J. E., xe2x80x9cInorganic Chemistry,xe2x80x9d 3rd Ed., HarperCollins: New York, 1983; pp. 726-731. In a concentrated aqueous sodium hydroxide solution, for example, at reduced temperatures (e.g., from about xe2x88x9240xc2x0 C. to about 0xc2x0 C.), borane undergoes disproportionation which occurs in two steps (i) asymmetric cleavage into a sodium borohydride molecule and a BH2+ fragment which coordinates two hydroxides; and thereafter (ii) disproportionation of the BH2+ fragment into an additional sodium borohydride molecule and a sodium borate molecule. The disproportionation is dependent on the anionic base and is independent of the cation moiety.
Replacement of water with a nonaqueous polar solvent or an aprotic solvent such as diglyme to minimize competitive hydrolysis of the BH2+ fragment allows the disproportionation to be achieved with higher efficiency, thereby providing greater yield. Examples of suitable aprotic solvents include: hydrocarbons, such as mineral oil, hexane or heptane; amides, such as dimethylacetamide; ethers, preferably organic glymes, such as diglyme and tetra(ethylene glycol) dimethyl ether (tetraglyme). Water can also be replaced with a nonaqueous polar solvent, such as methanol, ethanol, propanol, and isopropanol.